Tread depth measurement

ABSTRACT

A method of generating a three-dimensional topological surface representation of a tyre on a vehicle, the method comprising: using a tread depth measurement device to record tread depth data for a tyre surface moving relative to the tread depth measurement device; generating a movement profile of the tyre surface; and using the movement profile of the tyre surface to map the tread depth data onto a base tyre structure, thereby generating a three-dimensional topological surface representation of the tyre.

This application is a 35 U.S.C. § 371 national phase filing ofInternational Application No. PCT/GB2017/051156 filed on Apr. 25, 2017,and claims the benefit of United Kingdom Patent Application No.1607164.9, filed on Apr. 25, 2016, wherein the disclosures of theforegoing applications are hereby incorporated by reference herein intheir respective entireties.

The invention relates to assessment of vehicle tyre tread, andparticularly but not exclusively to three dimensional imaging of vehicletyre treads to obtain tread depth information.

Tyres for road-going vehicles are typically provided with a pattern ofgrooves, known as tread, for displacing water from between the tyre andthe road surface in order to improve traction in wet conditions.National laws typically prescribe minimum tread depths for the tyres ofroad-going vehicles for safety reasons. It is therefore important to beable to inspect a tyre tread to ensure that the tread has not becomeunsafe or illegal due to wear on the tread.

As purchasing and fitting replacement tyres can form a significant partof the total cost of running a vehicle, premature replacement can beregarded as wasteful. This is especially true for fleet operators, suchas a bus and haulage company, who have large numbers of tyres to replaceeach year. Conversely, continuing to use tyres that do not satisfy legalor commercial minimum tread requirements may be illegal and cancompromise vehicle safety. Thus the Applicant has identified a need fora system for easily monitoring the wear of a tyre, e.g. to determine howsoon the tyre will need replacing, or to identify any other flaws thatmay be unsafe.

A tyre's tread may be scanned in a number of different ways to measurethe tyre wear and tread depth. For example, optical scanning devicesincorporated in drive-over ramps, rolling roads or handheld scanners maybe used. However, conventional methods of using this kind of apparatusonly obtain a depth measurement corresponding to a line across the tyresurface (referred to as a “two dimensional” measurement, where the firstdimension corresponds to the depth of the tread and the second dimensioncorresponds to the length of the line). These known methods aretherefore potentially unreliable as they provide incomplete informationregarding the quality of a tyre's tread. For example, if there is aportion of the tyre that is badly worn, this could be very easily missedif the line or lines that are scanned fall outside the worn area.Similarly, if foreign bodies, such as dirt, debris or water droplets,are present in the tread grooves where the laser line intersects thetyre, then the resulting tread depth measurements would be inaccurate.

According to the invention there is provided a method of generating athree-dimensional topological surface representation of a tyre on avehicle, the method comprising:

-   -   using a tread depth measurement device to record tread depth        data for a tyre surface moving relative to the tread depth        measurement device;    -   generating a movement profile of the tyre surface; and    -   using the movement profile of the tyre surface to map the tread        depth data onto a base tyre structure, thereby generating a        three-dimensional topological surface representation of the        tyre.

The invention thus provides a method for mapping tread depth informationonto a base tyre structure (e.g. having curvature corresponding to thecurvature of the tyre), thereby to create a three dimensionaltopological surface representation of the entire tyre surface (wheretread depth data for the entire tyre is obtained) or of a part of thetyre surface (e.g. where tread depth data for a segment of the tyre isobtained).

The base tyre structure may be based on any non-linear equation that isan approximation to the shape of a tyre, or the shape of part of a tyre.For example, the base tyre structure could be based on an equation of anellipse or an equation of a circle if a large part of the circumferencehas been measured. An equation of an ellipse may be used for a tyre thatis deformed due to the weight of the vehicle, e.g. under strain. Ifdeformation is negligible or not present, the equation of a circle maybe used. An equation of a parabola may be used if a small segment of thetyre has been measured.

The method of the invention can, at least in preferred embodimentsthereof, provide more complete information that can be more easilyassessed to determine tread quality. The tread depth measurement devicemay be able to obtain tread depth data for portions covering all or asegment of the tyre.

The tread depth measurement device may be arranged to measure treaddepth using any suitable method. For example, the device may comprise atactile sensor (e.g. a series of fine fingers), an x-ray sensor, acapacitive sensor, etc. However, in a set of embodiments, the devicecomprises an optical sensor. For example, the device may comprise one ormore lasers to direct a pattern, e.g. a line, onto the tyre surface, andone or more cameras arranged to image the resulting laser pattern on thetyre surface. The tread depth measurement device may comprise a lasertriangulation sensor comprising a laser source and a 2D digital imager.As another example, the measurement sensor may comprise stereoscopiccameras with two 2D digital imagers and an LED or other light source inthe visible spectrum.

In a set of embodiments, the method comprises recording tread depth dataas the tyre turns, preferably at a constant rate, relative to the treaddepth measurement device while the vehicle is stationary. For example,this may be achieved while the vehicle is on a rolling road. In suchembodiments, the movement profile may be determined simply from therotation speed of the tyre which allows tread depth measurement datathat is obtained from the tyre surface to be mapped in a straightforwardmanner to a base tyre structure by relating the time at which the treaddepth data was recorded to a corresponding position on the base tyrestructure.

In another set of embodiments, the tread depth data is obtained using ahandheld device, e.g. which may be rolled across or around the surfaceof the tyre. In a set of such embodiments the device is adapted togenerate a movement profile by measuring its own movement relative tothe tyre, e.g. using a rotary encoder or the like. This also allows themovement profile to be obtained in a straightforward manner. In suchembodiments, the handheld scanning device could further comprise anaccelerometer. Feedback from the accelerometer may then be used todetermine the curvature of the tyre, which may be used in generating thebase tyre structure.

In a set of embodiments however, the method comprises recording thetread depth data as the vehicle moves towards or away from the treaddepth measurement device. For example, the tread depth data may berecorded as the vehicle moves towards or away from a drive-over ramp, oras it rolls up onto the ramp or down off of the ramp. Some of the treaddepth data could be obtained as the tyre rolls over the top of adrive-over ramp, e.g. from a portion of the tyre that contacts the ramp.However, typically the tread is squashed in the region contacting theramp, and so any tread depth measurement performed in the contact regiontends to be reduced, so this approach is not preferred.

When the distance between the tyre and the measurement device changeswhile the measurement is being taken, obtaining the movement profile canbe more complex. In such embodiments the distance could be determinedusing a separate sensing system, many examples of which are known, perse, in the art such as ultrasonic or laser range finders, radar Dopplerdetectors or the like. In a set of embodiments however the tread depthmeasurement device is arranged to use a sensing arrangement both fordetermining a distance to the tyre surface and for measuring treaddepth.

Thus in some embodiments, the movement profile comprises or is derivedfrom a set of data representing the positions at given times traced by apart of the tyre surface as the tyre is moving—referred to hereinafteras a “trace”. It will be appreciated that the part of the tyre surfacewhich is traced will not refer to a fixed physical point (e.g. aparticular patch of tyre tread) as it rotates around the tyre, butrather it refers to a location on the surface of the tyre relative tothe vehicle body, disregarding rotation. For example, the relevant partof the tyre surface may be the forward-most or rearward-most point onthe tyre relative to the direction of motion of the vehicle at any giventime, or it may be the part of the tyre surface that is closest to thetread depth measurement device. As an example, for a vehicle movingtowards a drive-over ramp at constant speed, the distance of a givenpart of the tyre from the measurement device will be a linearlydecreasing plot.

There are numerous environmental factors that can interfere with theacquisition of a complete and accurate movement profile. There are alsoaspects of the tyre structure that can prevent an accurate and completemovement profile, e.g. a complete trace, being obtained. For example, asthe tyre rotates while the trace is being acquired, the position beingtraced may coincide with a region of the tyre corresponding to tyrefeatures such as tread grooves, sipes or a tyre shoulder. The distanceto the tyre recorded at this point may be inaccurate or may containdiscontinuities due to the presence of the groove, sipe or tyreshoulder. In a set of embodiments, the method comprises discardingportions of the trace corresponding to tread grooves, sipes and/or tyreshoulders.

Any dirt, debris or water on the tyre tread surface may also causediscontinuities in the trace and may also introduce noise. Accordingly,in some embodiments, the method comprises filtering noise from thetrace. The method may also comprise smoothing the trace, e.g. to smoothout discontinuities therein.

As mentioned above, some portions of the trace may be disregarded orotherwise missing as a result of dirt, debris, tyre features, etc. Insome embodiments, the method comprises using spline interpolation toremove discontinuities in the trace. For example, where a portion of thetrace is missing, spline interpolation may enable the gap to be filledwith an approximation of the shape that the trace that would have had ifthe dirt/feature had not been present to cause the discontinuity. It isthereby possible to obtain a more complete trace (and thus a morecomplete movement profile) allowing a better, e.g. more reliable andmore complete, three-dimensional topological surface representation ofthe tyre to be obtained.

In some embodiments, the method comprises obtaining a plurality oftraces, each corresponding to a respective position on the tyre surface.Each of the plurality of traces may be obtained in the same way asdescribed above. It will be appreciated that this may help to provide amore reliable or more accurate movement profile. For example, theplurality of traces may be combined (e.g. averaged, as described below)to obtain a single trace that is used as the movement profile.

The method may comprise extrapolating one or more traces correspondingto respective positions on the tyre surface to extend said one or moretrace to the same length as a further trace corresponding to a furtherposition on the tyre surface. For example, there may be one trace in theplurality of traces that is longer than the other traces, and the othertrace may be extrapolated to extend them to the longer length. It willbe appreciated that there may be a trace that extends beyond the othertraces at one end, but which is shorter than one or more other traces atthe other end. In that case, all the traces may be extended byextrapolation at at least one end so that all the traces are the samelength. It will be appreciated that by extrapolating the traces so thatthey are all the same length, the traces can be combined or compared toobtain a single movement profile corresponding to the full length of thetraces. It will be appreciated that being “the same length” may meanthat the traces (expressed as distance as a function of time) extendover the same range of time values, i.e. they all start at the same timevalue and all end at the same time value.

The method may comprise discarding any trace that shows more than adefined variation from the median of the other traces of the pluralityof traces. This may help to eliminate contributions to the movementprofile from a trace resulting from an unreliable measurement. Forexample, if a trace corresponds to a point on the tyre close to theedge, e.g. close to the tyre shoulder, it may not be as reliable fordetermining a movement profile. This step of the method allows suchtraces to be removed. It will be appreciated that a different criterionfor discarding a trace could be used from this step, e.g. variation froma mean or other average of the plurality of traces.

The method may comprise taking the average of the plurality of traces toobtain a movement profile comprising a single trace. Combining theplurality of traces to obtain a single trace may provide a more reliableor more accurate movement profile, e.g. by reducing the effect of anyrandom errors in the traces.

It will be appreciated that where more than one of the steps describedabove relating to discarding data corresponding to features such astread grooves, sipes and shoulders; filtering noise; smoothing a traceor traces; spline interpolation; and discarding traces deviatingsubstantially from the median is carried out, they may be carried out ina different order from the order they are mentioned above. One or moreof the steps may be omitted, and/or additional method steps may beincluded between these steps. However, in a set of embodiments themethod comprises carrying out these steps in the order given above priorto calculating an average of the traces.

However it is obtained—either directly from measurement of the relativerotation between the tyre and depth measurement device at a fixed mutualspacing (such as in the rolling road and hand-held device examplesabove), or by the more complex methods described when the vehicle istravelling towards or away from the sensor—once the movement profile hasbeen obtained, the tread depth data recorded by the tread depthmeasurement device can then be mapped on to the base tyre structureusing the movement profile to relate the time at which the tread depthdata of a portion of the tyre was recorded to a corresponding positionon the base tyre structure to generate the three dimensional topologicalsurface representation.

In a set of embodiments, said mapping is based on a frame rate of themeasurement device. For example the movement profile may be used todetermine the appropriate part of the base tyre structure to map thedata recorded for each frame.

The method may comprise correcting for an angle of a scan relative tothe tyre axis (i.e. the tyre's axis of rotation). For example, where ahandheld device is used to scan the surface of the tyre, if the handhelddevice is rolled in a direction that is not exactly in line with thetyre axis, or not exactly following the tyre circumference, the treaddepth data obtained may correspond to a region of the tyre that isskewed relative to the tyre axis. Correcting for the skew of the scanrelative to the tyre axis may comprise using an algorithm whichidentifies the edges of a scanned region to identify data characteristicof the sidewalls of the tyre (e.g. data dropout corresponding to thesidewalls). This may then be corrected using a trigonometric rotationfunction.

In some embodiments, the generated three-dimensional topological surfacerepresentation may be improved using a combination of spatial low passfilters and/or a bilateral filter for preserving the edges of the tyre.The spatial low pass filters and/or bilateral filter may be appliedafter the three-dimensional topological surface representation has beengenerated, for example, to improve the aesthetic appearance of therepresentation. For example, the spatial low pass filters and/orbilateral filter may remove any residual noise.

Certain embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 shows a vehicle approaching a drive-over ramp tread depthmeasurement apparatus in accordance with an embodiment of the invention;

FIG. 2 shows a schematic close up view of the tyre on the vehicle ofFIG. 1 as the vehicle drives over the ramp;

FIG. 3 shows a front view of the tyre of the vehicle of FIGS. 1 and 2 atthree different times, showing a light pattern projected on to the tyreby the tread depth measurement apparatus;

FIG. 4 shows a movement profile for a vehicle moving at constant speedtowards the ramp and a movement profile for a vehicle moving at constantspeed away from the ramp for the embodiment shown in FIGS. 1 to 3;

FIG. 5 shows three movement profiles corresponding to a vehicleaccelerating towards the ramp, a vehicle stopping on the ramp and avehicle decelerating towards the ramp;

FIG. 6 shows example traces corresponding to a part of the tyre as thevehicle of FIG. 1 approaches the ramp, showing gaps in the tracesarising from removal of data corresponding to tyre features and dirt onthe tyre;

FIG. 7 shows the traces of FIG. 6, indicating where noise occurs as aresult of dirt or debris on the tyre;

FIG. 8 shows an example plurality of traces obtained for a number ofparts of the tyre as the vehicle of FIG. 1 approaches the ramp;

FIG. 9 shows the traces of FIG. 8, indicating where spline interpolationis used to complete gaps arising from data that is disregarded;

FIG. 10 shows the traces of FIG. 9, showing extrapolation of the ends ofthe traces to make them the same length;

FIG. 11 shows the traces of FIG. 10, with one of the traces disregardeddue to substantial variation from the median trace;

FIG. 12 shows a movement profile corresponding to an average of thetraces shown in FIG. 11;

FIG. 13 shows recorded distance data representing a movement profile fora tyre moving away from a ramp;

FIG. 14 shows a portion of a curve representing an idealised tyre basestructure for a segment of a tyre;

FIG. 15 shows an alternative embodiment in accordance with theinvention, wherein a tyre is mounted on a rolling road;

FIG. 16 shows a further alternative embodiment, wherein a handheld treaddepth measurement device is used to obtain tread depth data and amovement profile for a tyre in situ on a stationary vehicle; and

FIG. 17 shows the handheld device of FIG. 16 in multiple positionsaround a tyre to allow a larger segment of the tyre to be analysed.

FIG. 1 shows a drive-over ramp 2 for obtaining a three dimensionaltopological surface representation of a tyre in accordance with thepresent invention. FIG. 1 shows a vehicle 4 with a front wheel 6 and arear wheel 8 having respective tyres 10 and 12. The vehicle 4 is movingtowards the ramp 2 in direction of arrow 14. The drive-over ramp 2contains a tread depth measurement apparatus 16 which emits a laser beam18 to project a pattern of light onto the front tyre 10.

FIG. 2 shows a close up view of the drive-over ramp 2 and front tyre 10as the vehicle 4 drives over the ramp 2. For clarity, only the outlineof the front tyre 10 is shown. FIG. 2 shows the position of the fronttyre at three different times. The earliest time is t₁, followed by t₂,and the latest time is t₃. At time t₁, the laser beam 18 is projectedonto a region A of the tyre 10.

At the later time t₂, the front tyre 10 has rolled forward closer to thetop of the ramp 2. As the tyre rolls, it rotates about its axis so thatthe region A is now lower than the point at which the laser beam 18 isnow incident on the tyre 10. Instead, a new region of the tyre surface Bhas rotated into line with the laser beam 18. Region B is thereforeilluminated. At the later time t₃, the tyre has rolled forward further,and a new region C has come into line with the laser beam 18. Region Cis therefore illuminated by the laser beam 18 at t₃.

In the embodiment shown, the laser beam 18 has the shape of a sheet oflight so that the pattern projected by the laser beam 18 is an elongatepattern. This is shown in FIG. 3, which shows a front view of the tyreat times t₁ (left), t₂ (centre) and t₃(right). At t₁, the laser beam 18projects an elongate pattern 20 at position A on the tyre. A camera (notshown) contained in the tread depth measurement apparatus 16 is directedat the tyre surface to image the area of the tyre surface on which thelight pattern 20 is projected. The light pattern 20 projected by thelaser beam 18 is a straight line, but when it falls on the tyre, someportions of the lines fall in grooves, sipes and other tyre features.The camera is positioned so that it does not view the tyre at 90° to thetyre surface, but at an angle to the surface normal (e.g. around 45°).As a result, when the pattern is imaged by the camera, the patternappears as a deformed line, as regions of the light pattern 20 that fallin grooves, sipes, etc. appear laterally displaced due to the cameraviewing angle. The extent of the lateral displacement can then be usedto infer the tread depth for region A.

At time t₂, the vehicle is closer to the ramp, and the wheel 10 hasrotated so that a new region of the tyre B is illuminated by the laserbeam 18. This is shown as an elongate pattern 22. The position of regionA which was previously illuminated is shown by dotted line 20′. Thecamera images the pattern 22 to obtain tread depth data for region B.

At time t₃, the wheel has rotated further and a new region C isilluminated by the laser beam 18. This is shown as an elongate pattern24. The positions of regions B and A are shown respectively as 22′ and20″. The camera images the pattern 24 to obtain tread depth data forregion C.

Thus it can be seen that as the vehicle 4 approaches the ramp 2,different regions of the tyre tread come into view of the tread depthmeasurement apparatus 16. It will be appreciated that only a few exampletimes t₁, t₂ and t₃ have been shown. However, in accordance with theinvention, the tread depth apparatus may image regions corresponding tomany more times so as to create a high resolution three-dimensionaltopological surface representation. For example, the number of linescans (i.e. the number of two-dimensional regions) imaged may be of theorder of 1000. The number of data points in each line that is imaged maybe in the region of a few hundred, e.g. 300 or 400 data points. Thethree-dimensional topological surface representation may thereforecontain hundreds of thousands of data points, e.g. around 300,000 datapoints. However, it will be appreciated that the three-dimensionaltopological surface representation may contain more or fewer data pointsthan this. For example, in embodiments requiring a faster can speed,fewer data points may be recorded in order to obtain and process thethread depth data more quickly.

Referring again to FIG. 2, as can be seen from the sequential times t₁,t₂ and t₃, as the vehicle approaches and drives over the ramp, thedistance from the tread depth apparatus 16 to the point at which thelaser beam 18 impinges on the tyre surface varies as the tyre 10approaches the ramp 2. The distance between the tread depth apparatus 16and the tyre surface is inferred to obtain a distance estimate for eachregion (A, B, C, etc.) that is imaged. The distance is inferred usingthe position in the image captured by the camera of the portion of thetyre illuminated by the laser. Thus with reference again to FIG. 3, itcan be seen that the pattern 20 will be further towards the top of theimage captured by the camera, whereas the pattern 22 will be slightlylower and the third pattern 24 lower still as a result of the tyre 10advancing. The analysis software uses a look-up table to relate thechange in position of the patterns 20, 22, 24 across the imaging fieldto a distance (since the change is non-linear). Although theoreticallythe relationship in the look-up table is dependent on the diameter ofthe tyre, the Applicant has found that for a given type of vehicle (car,bus truck etc) the effect is very small and can be ignored.

FIG. 4 is a distance-time graph showing movement profiles 25, 26 for twotyres on respective vehicles undergoing different motion. Each movementprofile 25, 26 comprises an idealised trace of the position of part ofthe respective tyre. The distance plotted on the y-axis is, for example,the distance between the tread depth measurement apparatus in the rampand the closest part of the tyre surface. The linearly decreasingmovement profile 25 is a movement profile for a vehicle moving towardsthe ramp 2 with constant speed. The other movement profile 26, which islinearly increasing, is for a vehicle moving at constant speed away fromthe ramp. It will be appreciated that the ramp 2 depicted in FIGS. 1 and2 can also be configured to project a laser pattern onto the rear tyre12 of the vehicle 4 as the vehicle 4 moves away from the ramp 2. Asmentioned above, the movement profile comprise idealised traces, but itwill be appreciated that in practice a vehicle will typically not movewith exactly constant speed towards or away from the ramp, i.e. actualmeasured traces will not be perfectly linear.

FIG. 5 shows the other possible movement profiles 28, 30, 32, eachcomprising an idealised trace. Movement profile 28 relates to a vehicle4 accelerating towards the ramp 2. Movement profile 30 is for a vehicle4 moving towards/up the ramp 2, stopping on the ramp 2, and continuingto move towards/up the ramp 2. Movement profile 32 is for a vehicle 4decelerating as it approaches the ramp 2.

In practice, it may not be possible to obtain a complete andsufficiently accurate movement profile directly from a singlemeasurement of the distance to the tyre surface, e.g. from a singletrace obtained. As mentioned above, due to the presence of tyrefeatures, such as grooves, sipes and tyre shoulders, there may beregions of a recorded trace which are missing, noisy, or otherwiseinaccurate or unreliable. Accordingly, regions corresponding to grooves,sipes, etc., or dirt/debris may be removed.

FIG. 6 shows three exemplary traces 34, 36, 38 that might be recorded inpractice for a vehicle or vehicles undergoing the same movementrepresented by the idealised movement profiles 28, 30, 32 shown in FIG.5. The traces 34, 36, 38 shown in FIG. 6 have had portions correspondingto tyre features (e.g. grooves, sipes, tyre shoulders) and dirt ordebris removed to leave gaps 40.

FIG. 7 shows three traces 34′, 36′ and 38′ representing the situation inwhich the recorded traces exhibit noise 42 resulting from dirt or debrison the tyre surface.

When the traces are processed to obtain the movement profile, the noise42 is removed from the data, e.g. to arrive at the traces 34, 36, 38 asshown in FIG. 6 (i.e. without noise).

It will be appreciated that the example traces shown in FIGS. 6 and 7represent three possible different movement profiles. In practice, theanalysis will be carried out one movement profile, corresponding to theactual movement of the vehicle 4.

FIGS. 8-12 show an example of how a plurality of traces obtained for thevehicle 4 moving towards the drive-over ramp 2 may be processed toobtain a movement profile. These could be obtained, for example, bydividing up the image of the laser light pattern laterally into fivesections (for example, using a transmission grating beam splitter) andcarrying out independent analysis (for the purpose of establishing themovement traces) representing each of the five sections.

FIG. 8 shows a plurality of traces 44 corresponding to the vehicle 4moving towards a ramp 2, stopping on the ramp 2, and then continuing todrive over the ramp 2. Each of the traces 44 corresponds to a differentlateral part of the tyre surface as mentioned above. Each trace containsgaps 46 corresponding to regions that have been removed because theycorrespond to tyre features or dirt/debris as discussed above withreference to FIGS. 6 and 7. Noise has also been removed as discussedabove with reference to FIG. 7.

The first stage of processing after the removal of noise is shown inFIG. 9. This stage involves using spline interpolation to join the gaps46 resulting from the removal of noise. The regions added by splineinterpolation are shown by dotted lines 48. It will be noted that thetraces 44 shown in FIG. 9 are not all of the same length.

FIG. 10 shows the next stage of processing, which is to extrapolate theends of the traces 44 so that they are all the same length. The regionsadded to the traces by extrapolation are shown by dotted lines 50.

As mentioned above, a plurality of traces were obtained, eachcorresponding to a different position on the tyre surface. The exampledescribed with reference to FIGS. 8-12 uses five traces but it will beappreciated that a different number of traces may be used. Inparticular, a much larger number of traces could be used if desired.Only five are shown in this example for clarity. The advantage ofmeasuring traces for a plurality of sections across the tyre surface isthat an average can be taken to provide a more reliable movementprofile, e.g. to reduce the effect of random errors.

It can be seen from FIG. 10 that four of the traces are close togetherwhile one traces 54 deviates from the median of the other four. FIG. 11shows the plurality of traces with the deviating trace 54 removed inrecognition that a trace that deviates substantially from the median islikely to be erroneous, e.g. it may correspond to a region very close tothe tyre sidewall. The remaining traces 52 are then averaged to obtain amovement profile. The movement profile 56 resulting from taking theaverage is shown in FIG. 12.

FIG. 13 shows a movement profile calculated from recorded data for avehicle moving away from a ramp. It should be noted that this hasdistance along the horizontal axis and time along the vertical axis. Inthe initial region 58 the vehicle is starting to move away from theramp. In the next region 60 the vehicle slows down as it moves towardsthe end of the ramp, and in the subsequent region 62, the vehicle speedsup again as it continues to move away from the ramp.

FIG. 14 shows a section of a curve 64 used to map the recorded treaddepth data onto a base tyre structure. As mentioned above, for a smallsegment of the tyre (e.g. 2 inches to 4 inches, or 5 cm to 10 cm), thecurvature of the tyre surface can be approximated using a parabola. Thecurve 64 shown in FIG. 14 is a parabola with the quadratic equationax²+bx+c=y.

To determine the equation of this idealised curve so that it can be usedfor a base tyre structure, coefficients a, b and c must be calculated.

If data for a larger portion of the tyre or for the whole tyre have beenobtained (e.g. for a stationary vehicle on a rolling road), the sameprocess can be followed by using an ellipse or a circle, which has thefollowing general equation (with a=b for a circle):

${\frac{\left( {x - h} \right)^{2}}{a^{2}} + \frac{\left( {y - k} \right)^{2}}{b^{2}}} = 1$

It has been found that an ellipse may give a better approximation to thecurvature of the tyre surface.

FIG. 14 shows parameters α and β. The parameter β corresponds to y-valueof the maximum of the parabola, and may be expressed in terms of themovement profile distance (i.e. taking the zero of the y axis to be atthe position of the tread depth measurement device). The value of β canbe selected as any convenient point from the movement profile. The valueof α can be selected based on how much of the tyre circumference hasbeen captured in order to obtain the desired amount of curvature in thefinal three-dimensional representation. A larger value of α gives agreater curvature in the parabola, so a small value of α may be used ifa small portion of the tyre circumference has been imaged, while alarger value of α may be used if a larger portion has been imaged, e.g.for 2 inches (5 cm), the value of α might be chosen to be a₁ mm, and for3 inches (7.7 cm) it can be chosen to be a different value a₂ mm, wherea₂>a₁. The value for α may be chosen based on a linear relationship withthe length of the tyre circumference region imaged, e.g. it may becalculated using an equation α=kc where c is the length of the tyrecircumference region measured, and k is a constant.

To obtain the equation of a parabola corresponding to a base tyrestructure for the tyre being measured, numerical values for a, b and care calculated. Three points on the curve, corresponding to x values x1,x2 and x3 (shown in FIG. 14), are selected to provide three equationsthat can be solved to obtain a, b, c for the selected values of α and β:ax1² +bx1+c=β−αax2² +bx2+c=βax3² +bx3+c=β−α

Solving for a, b and c gives the result:

$a = {- \frac{\alpha}{\left( {{x\; 3} - {x\; 2}} \right)\left( {{x\; 2} - {x\; 1}} \right)}}$$b = \frac{\alpha\left( {{x\; 1} + {x\; 3}} \right)}{\left( {{x\; 3} - {x\; 2}} \right)\left( {{x\; 2} - {x\; 1}} \right)}$$c = {\beta - {\alpha\left\lbrack {1 + \frac{x\;{1 \cdot x}\; 3}{\left( {{x\; 3} - {x\; 2}} \right)\left( {{x\; 2} - {x\; 1}} \right)}} \right\rbrack}}$

The values obtain for a, b and c provide the equation of the parabolathat can be used as an ideal curve representing a base tyre structureundergoing movement according to the movement profile.

In physical terms, x1, x2 and x3 are the distances between the laser andthe tyre surface at particular times, e.g. t1, t2 and t3 as shown inFIG. 2. The relationship distance x3−x1 (i.e. the distance that the tyrehas moved closer to the laser) and the length of the tyre region that isimaged (the distance around the circumference from A to C) isnon-linear. In general there will exist points x1 and x3 for which thedistance x3−x1 is approximately twice the distance from A to C aroundthe circumference. To simplify the calculations and mapping process,these values of x1 and x3 may be chosen. The value of x2 can beconveniently chosen as the mid-point between x1 and x3 (which, for smallsegments, corresponds approximately to the position of B).

The values of x1 and x3 may be conveniently chosen to be the start andend points of the movement profile.

Let this ideal curve be a function of distance in x direction, I(x), andlet the movement profile be expressed as M(x). Another function Mp(x)can be generated according to Mp(x)=S1·I(x)−S2·M(x)·S1 and S2 arescalars that are used to ensure that the movement profile M(x) and theideal curve I(x) are scaled to the same physical dimensions. Mp(x)represents the ideal shape of the tyre with the effect of the tyremovement removed. Tread depth data for each point sampled is then mappedonto the function Mp(x) to obtain the three-dimensional topologicalsurface representation of the tyre. For an ellipse, every point x wouldhave two corresponding points on the y axis to be mapped.

FIG. 15 shows an alternative embodiment in which tread depth measurementdata is obtained using a rolling road. The vehicle is stationary whilethe tyre on the vehicle rotates. The vehicle is omitted for clarity.FIG. 15 shows a tyre at three different times, t₁, t₂, and t₃, where t₁is the earliest time and t₃ is the latest time. The tread depthmeasurement apparatus 68 is used to project a laser pattern onto thetyre surface and to image the regions of the tyre surface where thelaser pattern is incident as the tyre rotates in the manner previouslydescribed. At time t₁, the tread depth measurement device 68 images aregion A on the tyre surface. Like in the embodiments shown in FIGS.1-3, the laser pattern projected onto the tyre and imaged by the treaddepth measurement device 68 is a line extending in a direction fromsidewall to sidewall of the tyre. As time elapses, the wheel 10 rotates.At time t₂, a new region B has moved into view of the tread depthmeasurement device 68. The laser pattern is projected onto region B andis imaged by the tread depth measurement device 68. As time elapsesfurther, the tyre rotates to bring a third region C into view of thetread depth measurement device 68 at time t₃. The laser pattern isprojected onto region C and is imaged by the tread depth measurementdevice 68.

A movement profile is then obtained for the tyre based on the speed ofrotation of the tyre and the frame rate of the tread depth measurementdevice 68.

The tread depth data obtained by the tread depth measurement device 68is then mapped onto a base tyre structure, using the frame rate andspeed of rotation to relate the time at which each region was imaged tothe position of that region on the tyre surface.

FIG. 16 shows an alternative embodiment for obtaining tread depth datato produce a three-dimensional topological surface representation inaccordance with the invention. FIG. 16 shows a front view of the tyre 10when it is mounted in the vehicle 4. Dotted line 72 indicates thevehicle's wheel arch. The handheld tread depth measurement device 70generates a laser beam 74 to produce an elongate pattern on the tyresurface in use. The handheld device 70 is moved in the direction of thearrow 76 across the surface of the tyre 10 from one sidewall 78 to theother sidewall 80. The handheld device 70 comprises guide wheels 82 anda rotary encoder (not shown) which is used to relate the distancetravelled over the surface of the tyre to the time at which a treaddepth measurement scan is obtained using the handheld device 70.

The movement profile is then determined from the movement of thehandheld device 70 as calculated from the signal generated by the rotaryencoder. This enables the generation of a three dimensional topologicalsurface representation corresponding to a strip of tyre surfaceextending from one sidewall 78 to the other sidewall 80.

FIG. 17 shows how the handheld device 70 can be moved to differentpositions on the circumference of the tyre 10 to repeat the processdescribed with respect to FIG. 16. Three possible positions P, Q and Rare shown in FIG. 17. It will be appreciated that the process can berepeated for fewer or more positions, for example enough to cover thetop half of the tyre surface.

A three-dimensional topological surface representation for a region ofthe tyre surface corresponding to each position P, Q and R is obtainedusing the movement profile determined using the rotary encoder signal,as described above. These representations are then stitched together tocreate a complete scan of a larger region of the tyre.

The curvature of the base tyre structure onto which these sections aremapped may be determined using feedback from an accelerometer in thehandheld device 70. For example, the accelerometer can be used todetermine if the handheld device 70 is angled due to the circumferentialcurvature of the tyre (e.g. at position R, the hand-held device 70 is ata greater angle to the horizontal than at position Q). The accelerometercan also be used to determine if there is curvature between thesidewalls of the tyre, e.g. at the tyre shoulders, and this may beincorporated into the movement profile when each three dimensionaltopological surface representation is being generated. The threedimensional topological surface representations are then mapped onto thebase tyre structure to produce a three-dimensional topological surfacerepresentation for a larger portion of the tyre 10.

The invention claimed is:
 1. A method of generating a three-dimensionaltopological surface representation of a tyre on a vehicle, the methodcomprising: using a tread depth measurement device to record tread depthdata for a tyre surface moving relative to the tread depth measurementdevice; generating a movement profile of the tyre surface; and mappingthe tread depth data onto a base tyre structure, wherein mapping thetread depth data comprises using the movement profile to relaterespective times at which the tread depth data were recorded tocorresponding respective positions on the base tyre structure togenerate the three-dimensional topological surface representation of thetyre.
 2. The method as claimed in claim 1, comprising recording thetread depth data as the vehicle moves towards or away from the treaddepth measurement device.
 3. The method as claimed in claim 1, whereinthe movement profile comprises or is derived from a trace defined by aset of data representing the positions at given times traced by a partof the tyre surface as the tyre is moving.
 4. The method as claimed inclaim 3, comprising discarding portions of the trace corresponding toone of more of: tread grooves, sipes and tyre shoulders.
 5. The methodas claimed in claim 3, comprising one or more of: filtering noise fromthe trace; smoothing the trace; and using spline interpolation to removediscontinuities in the trace.
 6. The method as claimed in claim 3,comprising obtaining a plurality of traces, each corresponding to arespective position on the tyre surface.
 7. The method as claimed inclaim 6, comprising extrapolating one or more traces corresponding torespective positions on the tyre surface to extend said one or moretrace to a length that is the same as a length of a further tracecorresponding to a further position on the tyre surface.
 8. The methodas claimed in claim 6, discarding any trace that shows more than adefined variation from a median of the other traces of the plurality oftraces.
 9. The method as claimed in claim 6, taking an average of theplurality of traces to obtain a movement profile comprising a singletrace.
 10. The method as claimed in claim 1, comprising recording treaddepth data as the tyre turns relative to the tread depth measurementdevice while the vehicle is stationary.
 11. The method as claimed inclaim 10, wherein the movement profile is determined from a rotationspeed of the tyre.
 12. The method as claimed in claim 10, wherein thethree-dimensional topological surface representation is generated bymapping the tread depth data onto the base tyre structure based on aframe rate of the measurement device.
 13. The method as claimed in claim1, wherein the tread depth data is obtained using a handheld device. 14.The method as claimed in claim 13, wherein the device is adapted togenerate a movement profile by measuring its own movement relative tothe tyre.
 15. The method as claimed in claim 13, wherein the handhelddevice comprises an accelerometer.
 16. The method as claimed in claim15, comprising using feedback from the accelerometer to determine thecurvature of the tyre.
 17. The method as claimed in claim 1, furthercomprising correcting for an angle of a scan relative to the tyre axis.18. The method as claimed in claim 1, further comprising improving thegenerated three-dimensional topological surface representation using oneor more of a combination of spatial low pass filters and/or a bilateralfilter for preserving the edges of the tyre.
 19. The method as claimedin claim 1, wherein the tread depth measurement device comprises anoptical sensor.
 20. The method as claimed in claim 1, wherein the treaddepth measurement device is arranged to use a sensing arrangement bothfor determining a distance to the tyre surface and for measuring treaddepth.